Merge branch 'dt/commit-preserve-base-index-upon-opportunistic-cache-tree-update'

When re-priming the cache-tree opportunistically while committing
the in-core index as-is, we mistakenly invalidated the in-core
index too aggressively, causing the experimental split-index code
to unnecessarily rewrite the on-disk index file(s).

* dt/commit-preserve-base-index-upon-opportunistic-cache-tree-update:
commit: don't rewrite shared index unnecessarily

diff --cc builtin/commit.c
Simple merge

diff --cc t/t0090-cache-tree.sh
Simple merge